Cycles in Lake Beds of the triassic Sanford Sub-Basin of North Carolina
Lisa N. Hu, Daniel A. Textoris, 1994. "Cycles in Lake Beds of the triassic Sanford Sub-Basin of North Carolina", Tectonic and Eustatic Controls on Sedimentary Cycles, John M. Dennison, Frank R. Ettensohn
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Five wells were examined for evidence of cycles in the Middle Carnian Cumnock Formation. Strata in three wells, located in the lake depocenter, show strong periodicities which may be related to astronomical climate forcing as indicated by time series analyses using a modified Cooley-Tukey Fast Fourier Transform of gamma-ray logs from the three wells. The Butler well has strong signals at thicknesses of 4.2, 19.4, and 62 m. The Groce well has strong signals at 6.0, 25.6, and 61.5 m. The Hall well shows strong signals at 6.3 and 51 m. The thicknesses and ratios between them correspond to Van Houten cycles in outcrops of other Newark Supergroup strata. Periodicities may represent present-day 21,700-, 109,000-, and 412,000-year astronomical cycles.
Lithofacies sequences and petrology suggest expansion and contraction of the lake, and possibly correlate with the present-day 21,700- and 109,000-year cycles shown in the power spectra. If the 4.2-, 6.0-, and 6.3-m thicknesses represent the present-day 21, 700-year precession cycle, the sedimentation rates of strata in the three wells range from 0.19 to 0.29 mm of rock/yr, west to east. The life span of the lake was at least 1.2 million years. Renewed tectonic activity along the Jonesboro fault system to the east caused an increase in sedimentation rates in that part of the basin, masked lake cycles, and eventually eliminated the Cumnock lake.
Two other wells, located in the basin perimeter, do not display obvious cycles. The Dummit Palmer well, to the northwest, did not penetrate the entire Cumnock Formation and is affected by a diabase intrusive, faults, several coal beds and related basin-edge complications. The Gregson well, to the southeast, contains only minor lake-margin strata, and they were not amenable to any of our cycle analyses.
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Tectonic and Eustatic Controls on Sedimentary Cycles
The collected volume begins with a brief perspective by one of the conveners, followed by articles in order of increasing stratigraphic age. Eustatic sea-level changes and tectonic warpings of basins are competing mechanisms for explaining many stratigraphic patterns. The model for sea-level changes should be developed first for a basin, since it is allocyclic and leads to a series of time bands in the strata. The residual effects should then be modeled for tectonic patterns affecting the depositional processes. Doing the reverse limits time constraints on the tectonic warping models and will blur the resolution of detailed time surfaces in the strata. Case histories of situations with both tectonic warping and time surfaces marked by sea-level events will lead to improved interpretations of earth history.